matplotlib：使用emoji作为图例

概述

matplotlib主要通过代理对象实现自定义图例。根据官方文档https://matplotlib.org/stable/tutorials/intermediate/legend_guide.html，通过创建自定义类构造legend_artist方法即可实现emoj图例。其步骤大致为：

• 获取图例可视对象容器handlebox的位置信息。
• 构造图例可视对象。
• 将可视对象添加到图例可视对象容器handlebox中。

legend_artist方法的具体说明及源码如下。
https://matplotlib.org/stable/api/legend_handler_api.html#matplotlib.legend_handler.HandlerBase.legend_artist

def legend_artist(self, legend, orig_handle,
                  fontsize, handlebox):
    """
    Return the artist that this HandlerBase generates for the given
    original artist/handle.

    Parameters
    ----------
    legend : `~matplotlib.legend.Legend`
        The legend for which these legend artists are being created.
    orig_handle : :class:`matplotlib.artist.Artist` or similar
        The object for which these legend artists are being created.
    fontsize : int
        The fontsize in pixels. The artists being created should
        be scaled according to the given fontsize.
    handlebox : `matplotlib.offsetbox.OffsetBox`
        The box which has been created to hold this legend entry's
        artists. Artists created in the `legend_artist` method must
        be added to this handlebox inside this method.

    """
    xdescent, ydescent, width, height = self.adjust_drawing_area(
             legend, orig_handle,
             handlebox.xdescent, handlebox.y